The present invention relates to an improved neck structure of synthetic resin bottles.
Synthetic resin bottles are coming into increasing use in recent years. Particularly, polyethyleneterephthalate (PET) bottles, produced by biaxially blow-stretching a preform, are widely used due to their transparency, gas-barrier performance and strength.
However, PET bottles have a problem in thermal resistance. That is, they deform when filled with hot liquid such as juice after high temperature sterilization or when they are exposed to a hot shower or are immersed in a hot bath for sterilization after filling with, for example, carbonated natural juice.
Conventionally, to avoid this problem, biaxially stretched bottle drum bodies are heat set (Japanese Laid-Open Utility Model Publication No. SHO63-194115) or multiple-layered with a thermally resistant resin (Japanese Laid-Open Patent Publication No. SHO63-307054 and Japanese Laid-Open Utility Model Publication No. SHO63-194116).
The bottom of the bottle, which is hard to stretch, is treated for crystallization and blushing or is multiple-layered with a thermally resistant resin, thereby providing thermal resistance and rigidity.
The bottle neck is not stretched biaxially. The bottle neck is treated for thermal resistance using, for example, one of the following methods:
According to the inventions disclosed in Japanese Patent Publications Nos. SHO61-35056, SHO59-33101 and SHO54-68385, the bottle neck is provided with thermal resistance by thermal treatment for crystallization and blushing.
According to the inventions disclosed in Japanese Laid-Open Patent Publication No. SHO63-307054, Japanese Laid-Open Utility Model Publication No. SHO63-194115 and Japanese Laid-Open Patent Publication No. 61-259946, when a bottle preform is molded, thermal resistant resin and PET resin are co-injected to form a multiple-layer bottle neck, thereby providing thermal resistance to the bottle neck.
According to the invention disclosed in Japanese Laid-Open Patent Publication No. SHO58-149242, a bottle preform is molded in two steps: in the first step, a threaded outer cylindrical piece for a bottle neck is molded with a thermally resistant resin, and then the inner neck layer integral with the threaded cylindrical piece and the bottle drum body are injection-molded with bottle resin material, using the threaded cylindrical piece as a part of a neck mold. In this manner, a bore and bottle drum body are integrally formed with each other and with the threaded cylindrical piece.
According to the inventions disclosed in Japanese Laid-Open Patent Publications Nos. SHO54-72180 and SHO55-89056, a thermally resistant resin insert piece is molded in advance. A bottle neck is molded with bottle resin material surrounding the insert piece when the bottle preform is molded.
According to the inventions disclosed in Japanese Laid-Open Patent Publications Nos. HEI 1-124545 and HEI 1-118420, a thermally resistant resin core member, comprising a cylindrical body constituting the wall of a bottle neck and a flange constituting a support ring is molded in advance. The resin core member is set in the mold at the position for the bottle neck. The resin core member is embedded in bottle resin material so that the cylindrical body and the flange are sandwiched between bottle resin material layers when a bottle preform is molded. In this way, the resin core member provides thermal resistance to the bottle neck including the support ring.
According to the prior art disclosed in Laid-Open Patent Publication No. HEI 1-124545, a preform mold includes pins slidably projecting from and retracting into the mold. When a bottle preform is molded, the pins are projected to hold the resin core member within the mold.
According to the disclosure in Laid-Open Patent Publication No. HEI 1-118420, the resin core member includes a spacer projecting therefrom. The spacer is clamped in the mold to hold the resin core member in place before a bottle preform is molded.
According to the two foregoing inventions, bottle resin material flows on the inner surface the cylindrical body of the resin core member and is fed to the outer circumferential surface of the cylindrical body through holes formed positioned where the flange projects from the cylindrical body.
Conventional techniques for providing thermal resistance to a synthetic resin bottle, particularly to the bottle neck, as mentioned above, have a number of disadvantages as follows.
The disadvantages of the art disclosed in the Japanese Patent Publication No. SHO61-35056 and others in which the bottle neck is crystallized and blushed by thermal treatment include the following: The number of manufacturing processes is increased due to the thermal treatment process. The crystallized portions are brittle and exhibit degraded impact strength. The dimensional stability is degraded due to contraction. And the crystallized portions are not transparent.
The disadvantages of the art disclosed in Japanese Laid-Open Patent Publication No. SHO63-307054, and others that use co-injection of thermally resistant resin and PET resin to mold a bottle preform having a bottle neck with a two-layered structure, have the following disadvantages:
Due to the differences of the two resins in glass transition point, melting point and viscosity at a particular molding temperature, it is extremely difficult to control the flow rate of the two co-injected resin materials in the boundary of the two resin layers and to ensure appropriate resin flow to appropriate areas.
The disadvantages of the art disclosed in the Japanese Laid-Open Patent Publication No. SHO58-149242 in which a threaded outer piece for a bottle neck is formed separately, and of the art disclosed in the Japanese Laid Open Patent Publication No. SHO54-72180 in which a insert piece for a bottle neck is surrounded by thermally resistant resin, are as follows:
The piece may become separated from the bottle resin material due to the difference in thermal expansion between the piece and bottle resin materials. If the piece is joined firmly with the bottle resin material, the bottle neck may be cracked by the differential thermal expansion. Specifically, according to this prior art, the piece is joined to the bottle resin material only at the inner or outer side. Therefore, if hot liquid is poured into the bottle, the bottle resin may be separated from the piece due to the difference in thermal expansion between the piece and bottle resin materials.
The bottle neck obtained by the invention disclosed in the Japanese Laid-Open Patent Publication No. HEI 1-118420, in which a thermally resistant resin core member comprising a cylindrical body and a flange is embedded in the bottle resin material, has solved most of the problems of the above prior art. However, since the bottle resin material is led from inside to outside the cylindrical body through the holes positioned where the flange projects from the cylindrical body, the resin material outside the cylindrical body flows at a different rate from that inside the cylindrical body. As a result, differential pressure is produced between the outer and inner resin layers of the bottle neck. The differential pressure together with the heat of the bottle resin material itself causes the insert piece to be distorted, bent or deformed.